Mass spectrometry is the science of identifying the relative quantities of particles in a sample substance. Instruments for performing this analysis include mass spectrometers. Several types of mass spectrometers are presently in the prior art. Of these, the magnetic field mass spectrometer is the most popular.
The magnetic field mass spectrometer uses an ion source for providing an ion current comprising ionized particles of the sample substance. The ion current travels along a linear path into a magnetic field. The resulting electromagnetic force between the charged ionized particles and the electromagnetic field alters the linear path of the ionized particles, causing the ionized particles to travel arcuately through the magnetic field. The degree of arc through which the ionized particles travel is a function of the mass of each individual ionized particle, the velocity of each individual ionized particle, and the strength of the magnetic field. After traversing the magnetic field, the ionized particles resume traveling along a linear path. However, due to the arcuate displacement caused by the magnetic field, the linear path traveled by the ionized particles after traversing the magnetic field is angularly displaced from the linear path traveled by the ionized prior to entering the magnetic field. The degree of angular displacement of the linear path is a function of the degree of arcuate travel which is in turn a function of the mass and velocity of the individual ionized particle. The mass of the individual particles can thus be determined by determining the amount of the angular displacement.
To measure this displacement, the magnetic mass spectrometer includes a detecto. A common detector for the magnetic mass spectrometer comprises a photographic plate, with an emulsive coating. The photographic plate is positioned in the linear path of the ionized particles exiting the magnetic field. The ionized particles strike the photographic plate and activate the emulsion thereof. The photographic plate is thereafter develope to reveal a line for each mass of particle present in the sample substance. The relative density of the lines represents the relative quantities of the individual ionized particles in the sample substance. Alternatively, electrical means can be used to detect the angular displacement. A dynode of Faraday cup, can be used in plurality, or in combination with a moving slit, to detect the population of ionized particles at each angular displacement.
The popular magnetic mass spectrometers suffer from several known disadvantages. Primarily, these mass spectrometers use mechanisms for creating magnetic fields that are typically bulky and expensive to manufacture. Accordingly, such magnetic mass spectrometers are often large and expensive. Further, since magnetic mass spectrometers rely upon measuring angular displacement of the linear path of ionized particles, electronic detection used in conjunction with the magnetic mass spectrometer requires either plural detectors or moving parts to measure the physical displacement of the linear path of the ionized particles. These plural detectors, or moving parts, are also bulky and expensive ot manufacture. Accordingly, conventional magnetic mass spectrometers are not pratical for applications requiring small spectrometers at inexpensive production prices.
Other mass spectrometers which do not rely upon magnetic fields are referred to as radio frequency (RF) mass spectrometers. One type of RF mass spectrometer relies upon a four-pole structure wherein four conductive rods are positioned parallel to one another and spaced therefrom in a rectangular arrangement. The conductive rods are energized with an electrical signal that includes an alternating current (AC) component and a direct current (DC) component, thereby to create an electric field between the rods having respective AC and DC components. An ion current comprising ionized particles of the sample substance is provided from an ion source in the same manner as the ion current is provided in the magnetic mass spectrometer. The ion current from the ion source travels through the four-pole structure toward a detector. The frequency of the alternating current component of the electrical signal, and the magnitude of the direct current component of the electrical signal, are selected so that only ionized particles of a selected mass are permitted to completely traverse the four-pole structure. Ionized particles having a mass that is greater than the selected mass are attracted by the direct current component of the electric field so tht they collide with one of the conducting rods and do not traverse the four-pole structure. Ionized particles having a mass that is less than the selected mass are attracted to the conductive rods by the alternating current component of the electrical field and are also prevented from completely traversing the four-pole structure. The quantity of ionized particles exiting the four-pole structure is detected to determine the quantity of that ionized particle in the substance. Detection in this arrangement can be by means of a photographic plate, a single dynode, or a single Faraday cup.
The four-pole RF mass spectrometer also suffers from several known disadvantages. In the four-pole mass spectrometer, the length and spacing of the conductive rods is extremely critical to the operating tolerances of the resulting device. Accordingly, four-pole mass spectrometers are difficult and expensive to build. Further, these mass spectrometers are difficult to produce in large quantities and difficult to produce in smaller sizes. Still further, four-pole mass spectrometers do not provide good resolution for measuring particles having small mass. Accordingly, four-pole mass spectrometers are not acceptable for high-volume production of small mass spectrometers at inexpensive prices.
Another type of RF mass spectrometer that has been described in the literature relies upon linear acceleration to identify particles of selected masses. Unlike the nagnetic spectrometer and the four-pole mass spectrometer, these spectrometers require an ion source that provides an ion current at an extremely high velocity. The linear accelerator RF mass spectrometer includes an ion source similar to that of the magnetic mass spectrometer and the four-pole RF mass spectrometer. In addition, a D.C. accelerator is provided to receive the ion current exiting the ion source and to accelerate the ionized particles thereof to an extremely high velocity. The energy added by the D. C. accelerator is selected to be great enough so that the final velocity if the ionized particles is dependent almost entirely upon the ratio of the energy added by the D.C. accelerator to their mas, and not dependent on their initial velocity. Since all ionized particles have been elevated to the same energy level, the velocity of an individual ionized particle is a function of the mass of the ionized particle.
A series of equally-spaced drift tubes arranged in the form of a linear accelertor are positioned to receive the accelerated ionized particles. These drift tubes are each electrically conductive and include an interior channel the defines a path of travel for the ion current. Each drift tube is of equal length and is separated from its adjoining drift tube by an equal spacing referred to as a gap. An alternating current electrical signal is provided to the series of drift tubes to energize the drift tubes and create an electrical field in the gap intermediate successive drift tubes. Since the magnitude of the electrical signal is varying, the magnitude of the electric field created in the gap between adjacent drift tubes also varies. The frequency of the electrical signal so that portion of the ionized particles having the desired mass, and therefore a known velocity determined by their mass and energy level, will reach the gap between adjacent drift tubes when the magnitude of the electric field is at its maximum value. These ionized particles are referred to as synchronous particles. The magnitude of the electrical signal provided to the series of drift tubes, and similarly the magnitude of the electric field create within the gap, is selected so that the energy increase to any particle by successive exposure to the electric field is negligible. Conversely, ionized particles having a mass that is greater than, or less than, the desired mass will not enter successive gaps at the same time during each occurrence of the electrical field. Accordingly, these particles will be exposed to electric fields of various smaller levels, including retarding fields, i.e., an electric field that applies a force to the particle opposite to its direction of travel. The net result of the expsoure to electric fields of varying magnitude is to substantially decelerate ionized particles having a mass that is greater than, or less than, the selected mass. The quantity of particles of the desired mass is measured by detecting the quantity of ionized particles that maintain the initial high energy through the series of drift tubes. The detectors used by this drift tube mass spectrometer include an energy barrier having an energy level that is selected so that only the high energy particle is permitted to traverse the barrier. Accordingly, ionized particles that have a mass that is greater than, or less than, the selected mass, will decelerte when traversing the series of drift tubes and will not exit the drift tubes with sufficient energy to traverse the energy barrier. These particles will not be detected by the detector.
The linear accelerator RF mass spectrometer relies upon two critical assumptions, namely, that the velocity of the ionized particles exiting the accelertor is independent of their velocity entering the accelertor and, that negligible energy is added to the synchronous particles while traversing the series of drift tubes. Accordingly, the description of the linear accelerator RF mass spectrometer may not describe practical apparatus for high-volume production of an inexpensive mass spectrometer.
It is desirable, therefore, to provide an improved mass spectrometer tht is inexpensive to produce and which can be manufactured in volume. It is also desirable to provide an inexpensive mass spectrometer that can be produced in small sizes. It is further desirable to provide an improved method for mass spectrometry, which method can be performed inexpensively.